The present invention relates to a method of and apparatus for biasing magnetic material for the purpose of realizing the miniaturization of magnetic material employed in apparatuses such as transformers, inductors and the like.
The characteristic of magnetically soft material such as silicon steel, ferrite or the like is as shown by the hysteresis loop and saturation magnetization curve in FIG. 2. In the drawing, B is the magnetic flux density, Bm is the saturation magnetic flux density, Br is residual magnetic flux density, H is magnetizing force, Hc is coercive force and Hs is saturation magnetizing force. As is apparent from the saturation magnetization curve shown in FIG. 2, even if a magnetizing force H (designated by the product of current in the winding and the number of turns of the winding), which is in excess of a predetermined amount, is provided to the magnetic material, the magnetic flux density B within the magnetic material will not exceed a predetermined amount Bm, due to the magnetic saturation. The value Bm is determined by the property of the material and the configuration of the magnetic material, gaps in a magnetic path and the like. The higher the saturation value Bm is, the more miniaturized can be an inductor which can realize predetermined inductance value and conductive current value, or the more miniaturized can be a transformer which can be made by a small number of windings.
Now, in case that current flows or voltage is applied intermittently to one end of the winding wound about the magnetic core, the magnetic material fulfills its function in a range from the saturation magnetic flux density Bm to the residual magnetic flux density Br in FIG. 2. In case that the current is directed or the voltage is applied to one end of the winding, the magnetic material cannot be used beyond the range (Bm-Br) simply by providing the core having the winding wound on it as in the case of the conventional magnetic core.
However, as is apparent from FIG. 2, as a property of magnetic material, the magnetic flux density thereof can vary within a range from +Bm to -Bm. A magnetic core with magnetic polarity 10 shown in FIG. 3 has been devised based on this fact. The core has a permanent magnet 12 which is inserted into a magnetic path such that the magnetic flux density obtained when the magnetizing force H is "0", that is, the residual magnetic flux density Br, exhibits a negative value. Thereby it spreads substantially the usable range of the magnetic flux density. Thus, the combination of the magnetic core with a permanent magnet permits a reduction in utilizing volume of the magnetic material and hence the miniaturization of the apparatus.
However, the biasing method which uses the permanent magnet 12 for magnetically biasing (the magnetic core with magnetic polarity 10) has the following drawbacks: the core 10 is specific and expensive; the bias level is fixed because the bias is applied by the permanent magnet 12; the magnetic property of the permanent magnet 10 deteriorates with long term usage; and the like.